Method and equipment for detecting pattern defect

ABSTRACT

An inspection apparatus and method includes a light source, an illuminating unit having a polarization controller and an object lens for illuminating a specimen with light emitted from the light source and passed through the polarization controller and the object lens, a detection unit having a sensor for detecting light from the specimen illuminated by the illuminating unit, a processor which processes a signal output from the sensor so as to detect a defect on the specimen, and a display which displays information output from the processor. The processor processes an image formed from the signal output from the sensor in which the image is reduced in speckle pattern.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 12/271,348, filedNov. 14, 2008, now U.S. Pat. No. 7,791,725, which is a continuation ofU.S. application Ser. No. 11/593,091, filed Nov. 6, 2006, now U.S. PatNo. 7,456,963 which is a continuation of U.S. application Ser. No.11/184,981, filed Jul. 20, 2005, now U.S. Pat. No. 7,132,669, which is acontinuation of U.S. application Ser. No. 10/947,262, filed Sep. 23,2004, now U.S. Pat. No. 6,921,905, which is a continuation of U.S.application Ser. No. 09/473,296, filed Dec. 28, 1999, now U.S. Pat. No.6,800,859, the subject matter of which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

The present invention relates to a method and equipment for detecting apattern defect; and, more specifically, the invention relates to amethod and equipment suitable to detect and test a defect of a patternformed in a semiconductor wafer, a liquid crystal display, a photomask,etc.

Conventionally, detecting equipment, such as described in JapanesePublished Unexamined Patent Application No. 7-318326 (prior art No. 1),scans an image of a pattern under test (hereinafter referred to as a“test pattern” for simplicity) using an imager, such as a line sensoretc., and is able to recognized nonconformity as a defect by comparinggrayscale levels of the detected image signal to an image signal delayedby a prescribed time while moving the test pattern.

Moreover, a conventional technology concerning detection of a defect ina test pattern is disclosed in Japanese Published Unexamined PatentApplication No. 8-320294 (prior art No. 2). This prior art 2 is intendedto be applied to a test pattern on a semiconductor wafer etc. where ahigh density area of the test pattern, such as a memory mat part etc.,and a low density area of the test pattern, such as a peripheral circuitetc., exist in a mixed manner. This publication describes a methodcomprising the steps of: converting a digital image signal that isobtained through AD conversion of an image signal detected from theabove-described test pattern into grayscale levels so that thebrightness or the contrast ranging between the high density area and thelow density area of the test pattern is converted into a predeterminedrelation based on a brightness-frequency relationship of theabove-described detected image signal; performing a functionapproximation on both the image signal thus grayscale converted and animage signal to be compared (hereinafter referred to as a “comparisonimage signal”) therewith, which was also grayscale converted;integrating the difference between the two curves represented by thefunction approximations; aligning two image signals which were grayscaleconverted based on information of high-precision detection ofmisalignment obtained from the integral value; and detecting a minutedefect with high-precision by comparing test patterns while keeping thealignment between two image signals optimally.

Moreover, in the case of detecting a photomask, conventionally it hasbeen proposed that the light used in the detecting should be the same asthe exposure light so as to detect only a detrimental defect which willcause trouble in the actual exposure; accordingly, with this in mind, ithas been suggested that inspection of a photomask exposed withultraviolet light (hereinafter referred to as “UV light”) should beperformed using the same UV light as the exposure light. Patentapplications concerning this technology, as a technology to test theappearance of a circuit pattern on a photomask, include JapanesePublished Unexamined Patent Application No. 8-94338 (prior art No. 3)and No. 10-78668 (prior art No. 4).

In addition, a technology to measure the amount of phase shift in aphase shift mask is disclosed in Japanese Published Unexamined PatentApplication No. 10-62258 (prior art No. 5) and No. 10-78648 (prior artNo. 6).

Furthermore, a technology to clearly visualize a circuit pattern and aforeign material optically by inspecting a specimen with visible lightand UV light by making good use of a fact that materials used in aprocess have different absorption characteristics for visible light andUV light is disclosed in Japanese Published Unexamined PatentApplication No. 4-165641 (prior art No. 7) and No. 4-282441 (prior artNo. 8).

Moreover, means for measuring optically an external form of an objectusing an interferometer is disclosed in Japanese Published UnexaminedPatent Application No. 4-357407 (prior art No. 9), wherein UV light isapplied to the interferometer.

LSI fabrication in recent years has progressed toward finermicrofabrication in circuit patterns formed on wafers in response to aneed for high-integration, and a pattern having a width (feature size)as small as 0.25 μm or less is being required, reaching almost a limitof the available imaging optical systems. Therefore, efforts to attain ahigh NA in an imaging optical system and to apply the opticalsuper-resolution technology, as well as efforts to provide moresophisticated image processing, are being made. The above-describedprior arts 1 and 2 are directed to techniques that use those results.However, implementation of a high NA has already reached its physicallimit, and this measure has a problem of weakness for patterns having alarge pattern step height. Also, the optical super-resolution technologyand sophistication of image processing have a problem of limitedapplicability because of their non-linear response.

Therefore, an attempt to shorten the wavelength of light used in defectdetection, from a visible radiation region in conventional use to a UVlight region, is an essential approach.

On the other hand, the idea that the same light source as exposure lightshould be used, which has been originally devised for a photomask, iseffective for prior arts 5 and 6 for measuring the amount of phaseshift. This is because the amount of phase shift is directly linked withthe wavelength of the light source. However, in case defects are to bedetected by detecting the appearance of the whole surface of a testsample or a large area of a circuit pattern comparable to it, thetechnology wherein a wavelength of detecting light is chosen to be thesame as the exposure light (prior arts 3 and 4) is not necessarily anappropriate technique.

This is because the pattern transfer capability by exposure cannot bedetermined only by the wavelength of the light source and the conditionsof the optical system. The transfer capability is closely connected withvarious factors in a complicated way, such as the amount of exposure,properties of a resist, the amount of defocusing, an opticalcharacteristic of an underlying material, a developing process, etc.Consequently, the prior arts 3 and 4 are directed to techniques whichare suitable to analyze carefully the pattern transfer capability of asingle defect by performing a simulation including these complicatedconditions, but are different from a technology for detecting defects ofa large number of circuit patterns in a short period of time.

In the case where a large number of circuit patterns are examined in ashort time, it will be a practical solution for this problem tothoroughly detect any defects having a possibility of being transferredas a detectable defect with a sensitivity as high as possible by meansof a light source that is chosen only to detect defects, rather thanperforming a detection by applying an expensive, hard-to-handle exposurelight source.

In this case, since UV light is employed to improve the resolution,visible light that deteriorates the resolution cannot be employedjointly as is the case of the prior arts 7 and 8.

Further, since it is essential to perform a rapid detecting, a minutelyconverged laser beam as in the prior art 9 cannot be used. In the UVlight region, since a high-illuminance discharge lamp does not exist, ahigh-illuminance illumination by means of a laser is indispensable.However, as a result, when a laser beam is expanded to a whole field ofview, an interference fringe pattern due to interference of the laserbeam, a so-called speckle pattern, occurs and overshoot and undershootoccur in edge-portions of a circuit pattern, which make it impossible toobtain images.

Laser beams have excellent features as light sources. To use them in away which will give their features full play, when a certain area isilluminated, generally the laser beams are scanned using some kind ofscanning means.

For the scanning means, there are means capable of scanning by driving amirror mechanically to change a reflection direction, means capable ofscanning by applying an electric signal to an optical crystal to effecta change in diffraction direction or in refraction direction, and thelike.

Among the former means, there exist a galvano mirror, a polygon mirror(a polyhedron mirror), etc., and among the latter means, there exist anA/O deflector, an E/O deflector, etc.

Japanese Published Unexamined Patent Application No. 7-201703 disclosesequipment to scan a laser beam using a polygon mirror or a galvanomirror and to write a pattern with a minutely converged laser spot.Further, Japanese Published Unexamined Patent Application No. 8-15630discloses equipment to scan a laser beam using a polygon mirror or anA/O deflector write to a pattern with a minutely converged laser spot.Furthermore, Japanese Published Unexamined Patent Application No.10-142538 discloses equipment to write a pattern with a minutelyconverged laser spot in a scheme where two polygon mirrors are set insynchronization with each other, a phase difference of half the periodbetween mirror facets being set, to perform the scanning, and furtherthese polygon mirrors are switched between one another, so that acombination of polygon mirrors scans a laser beam with improvedefficiency. Furthermore, Japanese Published Unexamined PatentApplication No. 7-197011 discloses a device wherein two polygon mirrorsare stacked up with a phase difference of half the period between mirrorfacets thereof being set and a semiconductor laser diode is modulated insynchronization with its rotation. Furthermore, Japanese PublishedUnexamined Patent Application No. 5-34621 discloses a device whereinmirror facet angles of a polygon mirror are varied from facet to facet,so that two-dimensional beam scanning can be performed.

The scanning by a polygon mirror (polyhedron mirror) has a problem inthat since the scanning is performed under continuous rotation, thescanning is unavailable at the edges of mirror facets, the effectivescanning time is decreased, and as a result a decrease inefficiency isbrought about.

Moreover, if high-speed scanning is intended, a plurality of polygonmirrors cannot be rotated in synchronization with each other because ofthe continuous rotation. Therefore, it is impossible in a high-speedregion for a two-dimensional area to be scanned by integrating polygonmirrors, and for the scanning efficiency of polygon mirrors to beimproved in a scheme where two polygon mirrors are used to perform thescanning in synchronization with each other, also with a phasedifference of half the period between mirror facets thereof being set,and which are switched over for use, as disclosed in Japanese PublishedUnexamined Patent Application No. 10-142538.

Further, a method for directly modulating a laser, even in a schemewhere two polygon mirrors are stacked up with a phase difference of halfthe period between mirror facets thereof being set, as disclosed inJapanese Published Unexamined Patent Application No. 7-197011, is notsuitable for application in a deep ultraviolet wavelength region,because gas lasers and solid state lasers are not suitable for directmodulation. Further, one such laser is too expensive to make aconfiguration where a plurality of lasers are arranged and on-offswitched instead of being directly modulated.

Further, since a polygon mirror rotates continuously, the scanning rangecannot be changed. Therefore, the shape of the range scanned by apolygon mirror is limited to a rectangular area and hence polygonmirrors are not suitable to scan circular regions.

Further, in the wavelength region of deep ultraviolet light sources,there is a problem that a surface irregularity of a polygon mirrorcauses scattered light, which deteriorates the beam quality andrefection efficiency.

As for the scanning by galvano mirrors, ordinary galvano mirrors canonly perform low-speed scanning with a scanning speed of a few hundredHz at maximum, whereas a resonant-type galvano mirror can performhigh-speed scanning at a few kHz, but the driving signal is limited onlyto sinusoidal waves and the scanning angle varies sinusoidally, andtherefore the scanning speed of the beam is not constant. Because ofthis fact, when laser beam scanning is performed for illumination toobtain a detected signal especially using a storage-type sensor, thereis a problem in that a signal from a slowly scanned area is relativelylarge, whereas a signal from a fast scanned area is relatively small.

For E/O deflectors, there is no crystal usable in a wavelength range ofultraviolet to deep ultraviolet light. Therefore, current technologycannot respond to a request for a high-resolution optical system with alight source whose wavelength is shortened.

Moreover, for A/O deflectors there is only quartz for a crystal usablein the range of ultraviolet to deep ultraviolet wavelengths. However,the acoustic velocity in quartz is large. This fact is not a largeobstacle when an A/O element is used as a modulator, but becomes aproblem when it is used as a deflector. When a diffraction grating isformed in quartz using an acoustic element, a variation of the spacingbetween the gratings in response to a change of signal frequency appliedto the acoustic element is small because of its large acoustic speed.

This means that an angular region in which a deflector can deflect lightis small. Since the acoustic velocity in quartz is about 6000 m/s and anupper limit of the signal frequency that can be applied to an acousticelement is around 150 MHz, deflection of only 0.23 degree is achievableprovided that the variation range of frequency is ±100 MHz. Therefore,if a sufficient scanning range is intended to be achieved, an extremelylong optical path (1 m or more) should be provided. With provision of along optical path, there arises a problem of deterioration in beamposition and beam quality due to an environmental change, such asfluctuation of air in the optical path.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andequipment for detecting a minute circuit pattern rapidly with highresolution in order to solve the above-mentioned problems.

In addition, it is another object of the present invention to providedefect detecting equipment which is capable of detecting defects such assubmicroscopic foreign particles, a pattern defect, etc. by scanning ashort-wavelength (ranging from ultraviolet to deep ultraviolet) laserbeam for illumination over a test object such as a semiconductor waferetc. at a high speed with high-efficiency and detecting an optical imageof the test object.

To achieve the above-mentioned objects, the present invention adopts thefollowing steps of: employing a UV laser source as a light source;setting up means for suppressing the occurrence of a speckle pattern ofthe UV laser beam in an -optical path; and detecting an image of anobject by illuminating the surface of the object with the UV light whosecoherence was reduced.

More specifically, as a means for suppressing the occurrence of aspeckle pattern of the UV laser beam, one of the following means isintended to be provided: (1) means for converging rays of light from alight source onto a single point on a pupil of an objective lens andscanning the light thus focused on the pupil in exact timing with astorage time of a detector; (2) means for directing UV light emittedfrom the laser source into a bundle of fibers, each fiber of which isintentionally misaligned to the UV light, and converging rays of lightgoing out of the bundle of fibers onto the pupil of the objective lens;(3) means for directing the light into a group of fibers, to each ofwhich has a different length varied by the amount of the coherencelength of the laser source or more to other fibers, and converging raysof light going out of the group of the fibers onto the pupil of theobjective lens; and (4) means for illuminating the pupil with acombination of the above means.

In other words, the present invention provides pattern defect detectingequipment characterized by comprising: laser source means for emittingan ultraviolet laser beam; coherence reducing means for reducing thecoherence of the ultraviolet laser beam emitted from this laser sourcemeans; irradiating means for irradiating a sample with the ultravioletlaser beam whose coherence was reduced by the coherence reducing means;image detecting means for detecting an image of the sample irradiatedwith the ultraviolet laser beam produced by the irradiating means; anddefect detecting means for detecting a defect of a pattern formed on thesample based on information concerning the image of the sample detectedwith this image detecting means.

Further, the present invention provides a method of detecting a patterndefect characterized by comprising the steps of: emitting a laser beamwhose wavelength is not longer than 400 rim from a laser source;irradiating a sample with the emitted laser beam through coherencereducing means; detecting an image of the sample irradiated with thislaser beam; and detecting a defect of a pattern formed on the samplebased on information concerning this detected image of the sample.

Further, to achieve the above-mentioned object, the present inventionadopts a configuration wherein a set of polygon mirrors, which are madeup by stacking a plurality of polygon mirrors with the phase of mirrorfacets thereof mutually shifted, is rotated, a laser beam is modulatedthrough an A/O modulator in synchronization with rotation of theabove-mentioned polygon mirrors and is switched to either of the polygonmirrors appropriately to perform the scanning for irradiation, so thatthe object can be scanned at a high speed with a high efficiency evenwhen employing a short-wavelength laser beam that is needed to realize ahigh-resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a first embodiment of pattern defectdetecting equipment for a test pattern according to the presentinvention.

FIG. 2 is a diagram illustrating an emission spectrum of a dischargetube illumination.

FIG. 3( a) is a diagram showing the illumination condition on the pupilof the detecting objective lens and FIG. 3( b) is a diagram showing theillumination condition in the field of view, both produced by thedischarge tube illumination.

FIG. 4( a) is a diagram showing the illumination condition on the pupilof the detecting objective lens and FIG. 4( b) is a diagram showing theillumination condition in the field of view, both produced by laser beamillumination; FIG. 4( c) is a diagram showing a pattern in the field ofview; and FIG. 4( d) shows a detected signal produced from the patternof FIG. 4( c).

FIG. 5( a) is a diagram showing the illumination condition on the pupilof the detecting objective lens and FIG. 5( b) is a diagram showing theillumination condition in the field of view, both produced by laser beamillumination expanded on the pupil.

FIGS. 6( a) and 6(b) are diagrams showing illumination conditions on thepupil of the detecting objective lens and FIGS. 6( c) and 6(d) arediagrams showing the illumination conditions in the field of view,respectively, both produced by laser beam illumination according to thepresent invention.

FIG. 7 is a diagram showing a relationship between a CCD image sensordetector and an illuminated region in the field of view.

FIG. 8 is a diagram showing a relationship between the CCD image sensordetector and the illuminated region in the field of view to improve theilluminance.

FIG. 9( a) is a diagram showing the CCD image sensor and theillumination condition on the pupil of the detecting objective lens andFIG. 9( b) is a diagram showing the illumination condition in the fieldof view, both produced by laser beam illumination according to thepresent invention.

FIG. 10( a) is a diagram showing a TDI image sensor and the illuminationcondition on the pupil of the detecting objective lens and FIG. 10( b)is a diagram showing the illumination condition in the field of view,both produced by laser beam illumination according to the presentinvention.

FIG. 11 through FIG. 15 are diagrams schematically showing respectivecontrivances that reduce the spatial coherence of laser beamillumination according to the present invention.

FIGS. 11( a) and 11(b) are diagrams showing examples of the oscillationspectrum of a laser beam.

FIGS. 12( a) through 12(e) are diagrams showing different distributionsof lateral modes of outgoing light from a plurality of optical fibersilluminated by a laser beam.

FIG. 16 is a diagram showing a first embodiment of a laser beam scanningmechanism according to the present invention.

FIG. 17( a) is a diagram showing a second embodiment of the laser beamscanning mechanism according to the present invention.

FIG. 17( b) is a diagrammatic top plan view of an object being scannedby the mechanism of FIG. 17( a).

FIG. 18( a) is a diagram showing a third embodiment of the laser beamscanning mechanism according to the present invention.

FIG. 18( b) is a diagrammatic top plan view of a wafer scanned by alaser beam scanning mechanism.

FIG. 19 is a diagram illustrating a configuration of an irregular-typepolygon mirror employed in the third embodiment.

FIG. 20 is a diagram illustrating a condition of the light reflectedfrom the polygon mirror.

FIG. 21 is a diagram showing a fourth embodiment of the laser beamscanning mechanism according to the present invention.

FIG. 22 is a diagram showing a fifth embodiment of the laser beamscanning mechanism according to the present invention.

FIG. 23 and FIG. 24 are diagrams showing a variant and another variantof the fifth embodiment of the laser beam scanning mechanism accordingto the present invention, respectively.

FIG. 25 is a diagram showing a sixth embodiment of the laser beamscanning mechanism according to the present invention.

FIG. 26 and FIG. 27 are diagrams showing a variant and another variantof the sixth embodiment, respectively.

FIG. 28 is a diagram showing another embodiment of the laser beamscanning mechanism according to the present invention.

FIG. 29 is a diagram showing a second embodiment of the pattern defectdetecting equipment equipped with the laser beam scanning mechanismaccording to the present invention.

FIG. 30 is a block diagram showing a signal processing system in thepattern defect detecting equipment shown in FIG. 29.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, a first embodiment of a method and equipmentfor detecting a pattern defect in a test pattern according to thepresent invention will be described. FIG. 1 is a block diagram showing afirst embodiment of pattern defect detecting equipment according to thepresent invention. Numeral denotes an X, Y, Z, and θ (rotation) stage,on which a semiconductor wafer 1, an example of a test pattern, ismounted. Numeral 7 denotes an objective lens. Numeral 3 denotes anillumination light source (UV laser source) for illuminating thesemiconductor wafer 1, which is an example of a test pattern. Numeral 5denotes a polarizing beam splitter, which is constructed so as toreflect the illumination light from the illuminating light source 3,make it pass through the objective lens 7, and perform bright fieldillumination on the semiconductor wafer 1. Numeral 6 denotes a quarterwavelength plate, which forms a high-efficiency half mirror inconjunction with the polarizing beam splitter 5. Numeral 4 denotes ascanning mechanism for scanning a laser beam from the light source overthe pupil of the objective lens 7. Numeral 8 denotes an image sensor foroutputting a grayscale image signal according to the brightness(grayscale) of reflected light from the semiconductor wafer 1, which isan example of a test pattern. Numeral 9 denotes an AD converter forconverting the grayscale image signal obtained from the image sensor 8into a digital image signal.

While the stage 2 is being moved in a scanning mode and thesemiconductor wafer 1, which is an example of the test pattern, is beingmoved at a constant speed, information of the illuminance (grayscaleimage signal) of the test pattern formed on the semiconductor wafer 1 isdetected with the image sensor 8.

Numeral 10 denotes a grayscale converter for performing such grayscaleconversion as is described in Unexamined Japanese Patent Application No.8-320294 on the digital image signal outputted from the AD converter 9.In other words, the grayscale converter 10 compensates an image, whereunevenness of the illuminance has occurred due to thin film interferenceof the illumination light caused by a thin film formed on thesemiconductor wafer by the process by performing logarithmic conversion,index transform, polynomial transform, etc. The grayscale converter 10is configured so as to output a digital signal in, for example, 8 bits.Numeral 11 denotes a delay memory for storing and delaying an outputimage signal from the -grayscale converter 10 by the amount of 1 cell ora plural cell pitch, or 1 chip, or 1 shot, wherein a pattern on asemiconductor wafer comprises a plurality of these pattern units.

Numeral 12 denotes a comparator for detecting a defect by comparing theimage signal outputted from the grayscale converter 10, on which thegrayscale conversion was performed, to a delayed image signal obtainedfrom the delay memory 11.

The comparator 12 is used to compare an image outputted from the delaymemory 11 that was delayed by an amount corresponding to a cell pitchetc. to a detected image. By inputting coordinates of the arrangementdata etc. on the semiconductor wafer 1 beforehand, which are obtainedbased on design information, using an inputting means 15 consisting of akey board, a disk drive, etc., CPU 13 produces defect test data fromresults of comparison by the comparator based on inputted coordinates ofarray data etc. on the semiconductor wafer 1 and stores them in astorage device 14. This defect test data can be displayed on displaymeans such as display etc. according to need and also can be outputted.

By the way, the comparator may be as is described in Japanese PublishedUnexamined Patent Application No. 61-212708, for example, the comparatormay comprises an image alignment circuit, a difference image detectingcircuit for aligned images, a nonconformity detecting circuit forperforming a binary coded processing on the difference image, a featureextracting circuit for calculating areas, lengths (projected length),coordinates, etc. from binarized outputs, and the like.

Next, the light source 3 will be described. As described above, in orderto attain high resolution, it is necessary to employ a light sourcehaving a shorter wavelength. However, in wavelength regions of UV light(ultraviolet light) and DUV light (deep ultraviolet light), where thateffect can be maximized, it is rather difficult to have suchillumination with high illuminance. Regarding UV light sources, adischarge lamp is excellent. Especially, a mercury xenon lamp has astronger emission line in the UV region than other discharge lamps.

FIG. 2 is a diagram showing an example of a relationship of radiantintensity to versus wavelength for a mercury xenon lamp, indicating thatemission lines in the DUV region occupy only 1-2% of the total amount ofradiation, which is in direct contrast to the wide wavelength region ofvisible light in conventional use (i.e. occupying about 30% of totalamount of radiation). In addition, light emitted from a discharge lamp,whose radiation is not oriented in a particular direction, can be guidedonto a sample with a significantly small efficiency even in the case ofa carefully-designed optical system. After all, a sufficient amount oflight can hardly be secured with illumination of a discharge lamp in theUV region.

Moreover, when a discharge lamp having a large output is employed withthe intention to improve the intensity of illumination (theilluminance), since such a lamp slightly has a only larger size of aluminescent spot of radiation compared to those having a small output,the illuminance (light power per unit area) cannot be improved with sucha scheme.

Consequently, it can be rightly thought that an effective,high-illuminance illumination is optimally performed by a laser sourcewhose center wavelength is in the UV region or in the DUV region(hereinafter UV is used to indicate these two regions) which is notlonger than 400 nm, preferably not longer than 300 nm. The presentinvention provides means for solving such a problem when this UV laseris employed as a light source to illuminate the sample.

FIG. 3( a) is a diagram showing the illumination condition of the pupilof the objective lens and FIG. 3( b) shows that in the field of view ofthe sample when illuminated by normal white light. Here, AS denotes thepupil, while FS the field of view. At a position of the pupil, an image31 of the light source is formed; while, at a position of the field ofview, an almost uniformly illuminated area 32 comparable to the wholefield of view is formed.

Next, FIG. 4( a) is a diagram showing the illumination condition of thepupil of the objective lens and FIG. 4( b) shows that of the field ofview of the sample when illuminated by a laser source. In this case, theimage of the light source 41 at the position of the pupil is reduced toa point. A circuit pattern illuminated by illumination 42 in the fieldof view on the sample creates, for example, an image having such adetection waveform, as seen in FIG. 4( d) for a pattern whose crosssection is as shown in FIG. 4( c).

As can be seen from the drawing, an overshoot and undershoot occur inedge portions of a circuit pattern and a speckle pattern occurs when acircuit pattern is illuminated by a laser beam and its image is taken insuch that σ of the illumination is small. In other words, this canresult from the fact that illumination is not performed from variousangles in the field of view on the sample under the objective lens. Onthe contrary, in a normal white light illumination, illumination havinga certain size of its image on the pupil is produced, and illuminationis produced in the field of view on the sample from all directions at anangle comparable to the NA (the numerical aperture) of the objectivelens.

For any coherent light (light having the coherence) such as a laserbeam, σ (depending on an image size of a light source on the pupil) iszero. This is because, for coherent light, an image of the light sourceis a point, and therefore an image on the pupil is also a point. It goeswithout saying that, as shown in FIG. 5( a), using a different lenssystem, an enlarged beam of light 51 is made to project on the pupil.However, since a laser beam has coherence, there is obtained a similarresult 52 as is obtained for a case where all of the beam goes out of apoint where σ=0, accordingly the problem cannot be solved. Therefore,means for reducing the coherence of a laser beam is a prerequisite.Reducing the coherence requires reduction of either the temporalcoherence or the spatial coherence.

In view of this, the present invention proposes a method whereinfirstly, an image of the light source is formed on the pupil of theobjective lens of the detecting equipment, and then the image isilluminated on the sample through the objective lens in such a manner,for example, that first a point 61 in FIG. 6( a) is illuminated, alongwith a second point 62, a third point 63, . . . to achieve substantialillumination 65 all over the field of view, as seen in FIG. 6( c).During this process, the speckle pattern and an image of the overshootand undershoot can be obtained at each location, but respective imagesare not coherent to one another because these were obtained at differenttimes. Therefore, if these images are summed up on the detector, thesame image can be obtained as is obtained by an incoherent light source.To perform the summation on the detector, a storage-type detector suchas the CCD image sensor is suitable.

In this case, a scanning scheme may be spiral scanning 66, as seen inFIG. 6( b), and television-like (raster) scanning 67, as shown in FIG.6( d), and any other scanning may be employed, as long as the wholefield of view is scanned. However, it goes without saying that thescanning should be completed within a storage time of the detector.Therefore, it is recommended that the scanning should be performed insynchronization with operation of the detector.

In this way, an image by illumination 65 covering the whole field ofview as shown by FS in FIG. 6( c) can be obtained.

Further, although not shown in the drawings, the same effect as that ofthis scheme can be obtained through the steps of: making up a secondarylight source consisting of a plurality of point light sources byinserting a fly-eye lens in an optical path of a UV laser beam emittedfrom a laser source; forming an image of this secondary light sourceconsisting of a plurality of the point light sources on the pupil of theabove-described-objective lens; and varying a position of this image ofthe secondary light source temporarily on the pupil of the objectivelens.

Here, a case will be considered where a one-dimensional image sensor(for example, a solid state imager such as the CCD image sensor etc.) isused that is a storage-type detector and that is advantageous ineffecting rapid scanning of the test sample in the whole area of anarrow field of view such as that of a microscope. As shown in FIG. 7,when the whole area of the field of view is illuminated for aone-dimensional image sensor 71, illumination contributing to detectionis only that in area 72, whereas that in area 73 occupying a majorportion of the optical power does not contribute to detection at all. Toimprove the illuminance, it is desirable to perform linear illuminationas an area 82 to the one-dimensional image sensor 71, as shown in FIG.8. (A two-dimensional image can be obtained by scanning the CCD imagesensor in a direction perpendicular to an alignment direction ofelements of a sensor array thereof.)

In that case, by performing illumination whose longitudinal direction isset in the Y-direction as shown in FIG. 9( a) (a longitudinal directionof an area 91 shown by a bold solid line in the figure), illumination 92adjusted to a shape of the CCD image sensor 71 can be produced, as seenin FIG. 9( b). Also, the scanning on the pupil is performed in theX-direction. In this case, its scanning period T_(s) should be shorterthan the storage time T_(i) of the CCD image sensor. Through thisprocedure, summation of images can be done. A problem associated withthis scanning scheme is that, since the illumination has some spread inthe Y-direction on the pupil from the beginning, it is impossible toscan in the Y-direction. Consequently, the overshoot and undershootwhich occur in the Y-direction of the CCD image sensor in the field ofview cannot be reduced. On the contrary, if the length of theillumination in the Y-direction is shortened with the intention to scanin the Y-direction on the pupil, the width of the illumination in theY-direction become wider in the field of view and hence the illuminancedecreases.

To solve this problem, the present invention uses a Time DelayIntegration Image sensor (hereinafter referred to as a “TDI imagesensor”). The TDI image sensor, which is one of the CCD image sensors,has a structure in which a plurality of one-dimensional image sensorsare arranged in two dimensions and is of such a type that the amount oflight is intentionally increased by delaying, by a prescribed time, anoutput of each one-dimensional image sensor which takes a picture at aposition and then adding it to an output of an adjacent one-dimensionalimage sensor which takes a picture at the same position. In the case ofthe TDI image sensor, since N stages of the CCD image sensors (N equalsa few tens to one hundred) are aligned in the field of view, even if thewidth of the area illuminated in the field of view is widened by Ntimes, illumination light is utilized effectively for detection.

Because of this fact, the length of converged rays of light 102 in theY-direction on the pupil can be reduced to approximately 1/N times thelength of the case of the CCD image sensor, and the scanning can beperformed both in the X-direction and in the Y-direction on the pupil.Consequently, the overshoot and undershoot occurring both in theX-direction and in the Y-direction of the TDI image sensors on the pupilcan be reduced, and hence excellent detected images can be obtained.

Moreover, the scan period T_(s) on the pupil only needs to be shorterthan N times the storage time of one stage of the TDI image sensor.However, considering the illuminance distribution generated in the fieldof view, in order to attain substantially uniform detection, it isdesirable that T_(s) should be shorter than a half of N times T_(i).

Moreover, to perform uniform illumination in the field of view, it isdesirable that rays of light from a laser source should be convergedafter passing through a fly-eye lens or an integrator rather than havethese rays be converged directly onto the pupil.

Next, means for reducing the spatial coherence will be described. Toreduce the spatial coherence, it is only necessary to prepare aplurality of beams of light passing through optical paths whose lengthsare mutually different an the amount larger than the coherence length.More specifically, if output light of a laser is made to go into abundle of a plurality of fibers 11 or glass rods, each of which has amutually different length, as shown in FIG. 11, output light from such adevice will become incoherent light (having no coherence). If theselight components are arranged on the pupil, respectively, an image freefrom overshoot and undershoot and speckle can be obtained.

In addition, it is desirable with this scheme that the coherence lengthof the laser source should be shorter. For this end, a laser beam havingan oscillation spectrum with a plurality of longitudinal modes and hencea wide wavelength band Δλ₂ of emission wavelength, as shown in FIG. 11(b), is more suitable than a laser beam having an oscillation spectrumwith a single longitudinal mode and hence narrow wavelength band Δλ₁, asshown in FIG. 11( a).

Furthermore, regarding other contrivance for reducing the spatialcoherence, there is a scheme utilizing a phenomenon that, when light iscoupled to a misaligned fiber, lateral modes of outgoing light (spatialdistribution and optical intensity I to a space) vary from that of afiber with no misalignment. Normally, such variation of modes isregarded as an unfavorable phenomenon in industrial applications, andgenerally efforts to reduce the variation of lateral modes have beenexerted. However, in accordance with the present invention, in takingadvantage of this phenomenon another way, light from a laser is coupledto a plurality of fibers 1210 with their optical axes intentionallymisaligned to generate outgoing light having different distributions oflateral modes, as shown in FIGS. 12( a) through 12(e). As a result,since outgoing beams of light thus obtained become mutually incoherent,these beams are arranged on the pupil.

FIG. 13 is a diagram showing a condition where emitted light from alaser source 3 is divided into two beams of light 133 and 134 havingpolarization planes perpendicular to each other by a polarizing beamsplitter 131. Numeral 132 is a mirror for deflecting the light into adifferent direction.

Since two beams of light having mutually perpendicular planes ofpolarization are mutually incoherent, beams of incoherent light can beobtained with an optical system of a very simple configuration. Withthis scheme, only two beams of incoherent light can be obtained.However, if this scheme is combined with already-mentioned methods,light with virtually zero coherence can be easily obtained.

Further, since mutually independent light sources are incoherent,independent light sources 141, 142, 143, 144, . . . may be used, as theyare, to illuminate respective points on the pupil of the objective lens7, as shown in FIG. 14.

Furthermore, if this method is used in conjunction with the aforesaidmethod with the polarizing beam splitter, an effect that laser sourcesare substantially doubled in number can be obtained, as shown in FIG.15. Moreover, if the number of beams is maintained to the same asbefore, the number of laser sources can be reduced to one-half, andhence the cost can be held down.

In the foregoing, a plurality of means for reducing the coherence of aUV laser beam, illuminating 4 plurality of points on the pupil with thisUV laser beam with reduced coherence, and obtaining an image byconverging rays of light with the objective lens have been described.Each of these means can be used jointly with other means. Moreover, anyother means for reducing the coherence equivalent to these means may beused.

Moreover, although not shown in the drawings, a scheme where a diffuseris inserted on the way of an optical path of a UV laser beam and thisdiffuser is rotated or reciprocated may be used to reduce both thespatial coherence and the temporal coherence of the UV laser beam at thesame time. Further, this diffuser can be used in conjunction with othercoherence reducing means mentioned above.

As a scanning mechanism 4 of a laser beam indicated in FIG. 1, anarrangement shown in FIG. 16 through FIG. 28, which will be describedbelow, may be adopted. Firstly, referring to FIG. 16, a first embodimentof the laser beam scanning mechanism according to the present inventionwill be described. That is, in the first embodiment, polygon mirrors 108and 109 are stacked up with a rotation phase difference of half theperiod between respective mirror facets thereof being set and the stackthus made is configured to be rotatable by a rotating motor 101. Thepolygon mirrors 108 and 109 are mirrors of mutually identical shape andtheir effective scanning time ratio is approximately 50%. Here, theunavailable time is defined as a time when a laser beam hits an edgepart of each mirror facet of the polygon mirror and thereby speculareflection cannot be exerted, whereas the available time is defined as atime when the laser beam hits a flat plane to exert specula reflection.The available scanning time ratio is given by the ratio of the availablescanning time to the total time. Further, each polygon mirror is fixedon a spindle of the rotating motor 101 in such a manner that mirrorfacets comprising one polygon mirror are set with a rotational phase ofhalf the period shifted to corresponding mirror facets comprising theother polygon mirror.

At the same time, each of these polygon mirrors 108 and 109 areirradiated with a laser beam 104 from a laser source (not shown in thedrawings) through an A/O modulator 105. An A/O modulator 105 switchesthe laser beam into either of irradiation paths 106 and 107 based on acontrol signal generated by a rotation position-mirror switching signalconverter 103 from an output 102 transmitted from an encoder attached tothe rotating motor 101. In case two polygon mirrors are switched over,if first order light (for example, light 107) and zero-th order light(for example, light 106) from the A/O modulator 105 are used, theswitching can be done using only the single A/O modulator 105, and thisis convenient. It should be understood that since just switching anoptical path is needed, any means other than A/O modulators may he used.That is, during the available scanning time of the polygon mirror 109,the optical path in use is switched to the optical path 107, and duringthe unavailable scanning time of the polygon mirror 109, the opticalpath in use is switched to the path 106. By this scheme, the availablescanning time ratio of this polygon mirror system reaches approximately100%.

Therefore, even when a laser other than an easy-to-modulatesemiconductor laser is used along with a polygon mirror which has a highscanning speed and hence a low available scanning time ratio of about50%, highly-efficient and high-speed scanning of a laser beam can beperformed.

If the number of stages of polygon mirrors to be stacked is increased,this scheme can be applied to a polygon mirror whose available scanningtime ratio is shorter than 50%. In this case, two or more polygonmirrors are stacked up in such a way that the mirror facets comprisingeach polygon mirror are shifted by one n-th times the period (where ndenotes the number of the polygon mirrors) with respect to mirror facetsof other polygon mirrors.

By the way, in this configuration there is an optical system (forexample, a lens system) 110 having both: a function of a lens system 112capable of scanning two laser beams, which are reflected from thepolygon mirrors 108 and 109, respectively, along with the same scanningline on an object (in case such a defect as minute foreign particles, aminute pattern defect, etc. is examined, the object will be a testobject); and a function of a F-θ lens 111 capable of scanning both laserbeams with the same scanning speed.

Next, referring to FIG. 17( a), a second embodiment of the laser beamscanning mechanism according to the present invention will be described.The second embodiment comprises, as shown in FIG. 17( a), a polygonmirror 201 and a polygon mirror 202 which have different numbers ofmirror facets relative to each other and are stacked up. Because of thisconfiguration, each scanning angle range (i.e. angle range in whichreflected light is scanned), which is inversely proportional to thenumber of mirror facets, is different from that of the other polygonmirror. However, the diameters of the polygon mirrors 201 and 202 aredetermined so that a period of time required for each angle range to bescanned is the same.

This configuration provides a scanning scheme, in case the object 204having a circular shape, for example a semiconductor wafer, is moved ina direction indicated by an arrow 205 in FIG. 17( b) (on the plane ofthe figure) and the scanning is performed with polygon mirrors in adirection perpendicular to the direction of the arrow, wherein, in asection 208 where a small scanning angle is sufficient, the A/Omodulator 105 scans, for example, the zero-th order diffraction light206 using the polygon mirror 202, whereas, in a section 209 where alarge scanning angle is necessary, the A/O modulator 105 scans, forexample, the first order diffraction light 207 instead using the polygonmirror 201, as shown in FIG. 17( b).

With the use of this second configuration, the time necessary to scanthe whole surface of the object 204 can be reduced compared to that of aconfiguration wherein the scanning is performed with the same scanningangle which is only suitable for the area 209.

Here, a configuration using two kinds of polygon mirrors has beendescribed. However, it is well understood that if more kinds of mirrorsare used, highly-sophisticated illumination with higher efficiency canbe performed. Moreover, since the timing necessary for switching eachpolygon mirror is not so delicate, it is not necessary to detect theangle from an encoder attached to the rotating motor 101 and to rotatepolygon mirrors synchronously. Thus, polygon mirrors 201 and 202 may berotated using respective motors.

By the way, also in this second configuration, there is an opticalsystem (for example, a lens system) 110 having both: a function of alens system 112 capable of illuminating the same scanning line on theobject (in case a defect such as minute foreign particles, a minutepattern defect, etc. is examined, the object will be a test object) witha laser beam reflected by either of the polygon mirrors 201 and 202; anda function of the F-θ lens 111 capable of scanning with an identicalscanning speed.

Next, referring to FIG. 18( a), FIG. 18( b) and FIG. 19, a thirdembodiment of the laser beam scanning mechanism according to the presentinvention will be described. By the way, in case the scan object 204 ofa laser beam has a circular shape, such as a semiconductor wafer, if alaser beam is scanned simply over a rectangular area circumscribing thecircle with respect to the scanned object 204, the laser beam inevitablywill scan outside of the object area and hence the efficiency ofirradiation goes down. In view of this fact, this third embodimentadopts a polygon mirror 300 a, as shown in FIG. 18( a), which is a topview of the polygon mirror 300 a), wherein angles θ7 to θ1 and θ1 to θ7subtended by respective mirror facets 307 a to 301 a and 301 a to 307 a(namely, lengths of mirror facets along its circumference) are varied,respectively, and hence the scanning range (scanning length) on theobject L7 to L1 and L1 to L7 can be varied when the laser beam 104 (106,107; 206, 207) is reflected.

By the way, in case a laser beam 210 of, for example, a slit shape orspot shape is scanned, as indicated by L7 to L1 and L1 to L7, over thescan object 204 of circular shape, such as a semiconductor wafer, asshown in FIG. 18( b), since it is generally necessary to scan the objectalong equally spaced lines in the Y-direction, a subtended angle by eachmirror facet can be determined in such a manner that the object of acircular shape is scanned over its whole surface while the polygonmirror is rotated at least one revolution, even if a stage (not shown inthe drawings) on which the object 204 is mounted is translatedintermittently in the Y-direction. By the way, if the laser beam 210 isshaped in a slit form, a linear image sensor composed of a CCD imagesensor, or a TDI image sensor, or the like can be used as a detector.

Moreover, as shown in FIG. 19, a polygon mirror 300 b may be configuredso that mirror facets 307 b to 301 b and 301 b to 307 b have differentinclined angles α7 to α1 and α1 to α7, which are varied gradually, withrespect to a rotation axis, and hence, for example, the slit-like orspot-like laser beam 210 can be scanned two-dimensionally over theobject 204 of circular shape, as shown in FIG. 18( b). Further, numeralsL1 to L7 indicate scanning lines produced by mirror facets 301 b to 307b in FIG. 18( b).

Therefore, in the case of the polygon mirror 300 b, it is not necessaryto translate a stage with the object 204 of circular shape mounted on itin the Y-direction. However, when the object 204 is irradiated with thelaser beam 210, positing of the laser beam 210 is necessary.

In addition, this third embodiment can be applied to either of theabove-described first and second embodiments.

Next, referring to FIG. 20 and FIG. 21, a fourth embodiment of the laserbeam scanning mechanism according to the present invention will bedescribed. A polygon mirror 601, as shown in FIG. 21, is manufactured bycutting a block of aluminum, its alloy, or beryllium. Mirror facets ofthe polygon mirror 601 thus manufactured are occasionally ofinsufficient profile irregularity (plane roughness) when beingirradiated with a laser beam 602 whose wavelength is not longer thanthat of ultraviolet rays. FIG. 20 is an illustration showing a reflectedbeam in this case projected on a screen. As shown in FIG. 20, when theprofile irregularity of the mirror facet is insufficient, there occursscattered light 502 in the periphery of a specula reflection beam oflight 501 from the mirror facet. An experiment carried out by theinventors has revealed that when the plane-roughness of the mirror facetis controlled to be equal to 50 A or less, the amount of scattered light502, namely the fraction of the light lost, is reduced to 5% or less ofthe total amount of reflected light.

Since the scattered light 502 deteriorates the beam quality, it isdesirable to remove it as much as possible. In view of this, a fourthembodiment is configured so as to remove the scattered light 502 byproviding a scattered light trap 603, for example, a plate with a holethrough which only the specula reflection beam 501 passes. In this way,by providing the scattered light trap 603, the quality of the scanninglaser beam can be improved by removing the component of the scattered-light 502 not only in the case of a surface roughness of more than 50A, but also in the case of surface roughness of not more than 50 A.

By the way, the larger the clearance between the mirror facet of thepolygon mirror and the scattered light trap 603, the better will be theperformance in trapping the scattered light 502. An experiment carriedout by the inventors has revealed that a clearance of about 1 m can givea satisfactory trapping performance.

The fourth embodiment described above can also be applied to either ofthe above-described first, second, and third embodiments.

Next, referring to FIG. 22 to FIG. 24, a fifth embodiment of the laserbeam scanning mechanism according to the present invention will bedescribed. The fifth embodiment is constructed basically using a galvanomirror. Ordinary galvano mirrors can only perform a slow scanning with ascanning frequency in the range of up to a few hundreds Hz at best.However, a resonant-type galvano mirror, which is also called as aresonant galvano mirror, can perform a scanning in the range of a fewkHz or more, but reportedly, the driving signal must consist only ofsinusoidal waves. Therefore, the scan angle varies sinusoidally and thescanning speed of the beam cannot be constant.

Accordingly, when the laser beam 104 is scanned over the object 204mounted on the stage 130 using a resonant-type galvano mirror, called aresonant galvano mirror, and an image signal is obtained from an imageof the object 204 by using a sensor, especially a storage-type sensor,for example a TDI image sensor, the intensity of the image signal froman area being slowly scanned is relatively large, whereas that from anarea being fast scanned is relatively small, because of the varyingscanning speed of the laser beam.

In view of this, the fifth embodiment, as shown in FIG. 22, has aconstruction comprising a resonant-type galvano mirror (resonanceoperation-type mirror scanner), which is also called a resonant galvanomirror, wherein a scanning mirror 702 is driven by an actuator 701 basedon a sinusoidal driving signal 711 obtained from a sinusoidal signalsource 703, and a computer 705 reads the direction of the mirror from anencoder 704 attached to the actuator 701, finds the scanning speed atthe point where the scanning angle is read from the scanning angle thusread, and generates a control signal. Further, the computer 705 controlsan A/O modulator 706 based on a control signal 710 indicating thescanning speed. Specifically, when the scanning is slow, thetransmittance is decreased in proportion to it, whereas when thescanning is fast, the transmittance is increased, so that the quantityof light of a laser beam 707 which is made to hit the mirror 702 isbeing varied. As a result, even if the scanning speed of the laser beamwherewith the object 204 is irradiated varies, the object mounted on thestage 130 is irradiated with the laser beam 140 with an intensitycorresponding to the scanning speed and hence the intensity of an imageobtained from the object 204 becomes constant, accordingly, the value ofthe detected signal (image signal) becomes constant when detected by adetector 801, for example, a storage-type sensor, as shown in FIG. 23.By the way, numeral 121 denotes a half mirror and numeral 122 denotes anobjective lens.

Moreover, control of the laser beam using the A/O modulator 706 may beprovided after the reflection rather than before the reflection.However, in the case of after-reflection scanning, a laser beam isscanned instead.

Moreover, in the fifth embodiment, as shown in FIG. 23, means to changethe amplification ratio in order to control the signal (image signal)detected by the detector 801, based on the control signal 710 indicatingthe scanning speed which is outputted from the computer 705, can give adetected signal (image signal) 803 having a constant strength even whenthe scanning speed of the laser beam 140 with which the object 204 isirradiated varies.

Further, in the configuration shown in FIG. 23, the A/O modulator 706 isassumed to be used in a configuration as shown in FIG. 22, and thereforedescription of the A/O modulator 706 is omitted. However, othermodulating means capable of controlling the intensity of the laser beamin response to the scanning speed may be used without the use of the A/Omodulator 706 even when the scanning speed of the laser beam with whichthe object 204 is irradiated varies.

Moreover, since the direction of the mirror 702 varies with a constantphase delay with respect to the sinusoidal driving signal 711 suppliedfrom the driving signal source 703, the control signal 710 to be usedfor controlling the above-described transmittance and amplificationfactor may be a control signal from a computer 901, which performs acalculation so as to give a phase difference to the driving signal 711itself, as shown in FIG. 24, rather than an output from the encoder 704,as shown in FIG. 22 and FIG. 23.

The fifth embodiment described above is a scheme for keeping thedetected output constant electrically, and hence this method basicallyintroduces a loss and reduces the efficiency. Therefore, it is desirableto realize the scanning with a constant speed optically.

Next, referring FIG. 25 to FIG. 27, a sixth embodiment of the laserscanning mechanism according to the present invention will be described.The sixth embodiment in FIG. 25 represents a case where a lens-likeoptical element 720 is used. In the resonant scanner 701 (i.e. resonantoperation-type mirror scanner), a beam after reflection has a slowscanning speed (i.e. the variation of the deflection angle per unit timeis small) at an angular position giving a large deflection angle,whereas the beam has a fast scanning speed (i.e. the variation of thedeflection angle per unit time is large) at an angular position giving asmall deflection angle. In view of this, a curved surface of thelens-like optical element 720 is formed so as to have a larger curvaturewith increasing distance from the center so that at an angular positiongiving a larger deflection angle, the defection angle becomes evenlarger according to the deflection angle.

The sixth embodiment shown in FIG. 26 represents a case where thereflection direction is modified using a fiber 1101 in place of thelens-like optical element 720. In this fiber bundle 1101, the outerfibers have a larger degree of outward inclination to make a largerangle of deflection, in accordance with the position of the fiber on theexit side thereof, than an inner fiber.

In the embodiment of FIG. 27, the reflection direction is modified toconvert the direction using a holographic plate (or a deflectiveelement) 1201 instead of the lens-like optical element 720. Thisholographic plate 1201 is formed so as to have a grating whose spacingbecomes narrower with increasing distance from the center so that theoutgoing beam from the mirror which has passed through the holographicplate 1201 at a position nearer to its periphery suffers a largerdeflection.

The foregoing is a description of various embodiments which employ aresonant galvano mirror. However, these embodiments can be constructedwith any other resonant operation-type mirrors.

As described above, A/O deflectors can generate only a small amount ofscanning angle, but their high-speed scanning capability and ease ofcontrol remain attractive as before. In case such an A/O defector isused, to attain a necessary scanning range with a minute scanning angle,it is necessary to secure a long optical path after the light goes out.However, when the long optical path is provided in air, the beamposition and beam quality may deteriorate due to environmental changes,such as fluctuation of the air in the optical path, etc.

To circumvent this, as shown in FIG. 28, the optical path of the beamafter leaving the A/O deflector 1301 is folded in a compact spacemultiple times using to folding mirrors 1302 and 1303, and the space issealed from the outside to construct a folding optics unit. By isolatingthis unit thermally from the outside using an insulating material andproviding a heater for keeping the unit at a temperature higher than theouter environment, etc., problems of fluctuation in air and thermaldeformation of optical components used are evaded, providing an opticalpath which makes it possible to obtain a necessary scanning rangesecurely.

As described in the foregoing, the laser beam scanning mechanismaccording to the present invention makes it possible to performhigh-speed and highly efficient laser beam scanning even in thedeep-ultraviolet region.

Next, referring to FIG. 29, an embodiment of defect detecting equipmentequipped with a laser beam scanning mechanism selected from the firstembodiment to the fifth embodiment described above will be described.This embodiment is constructed with an epi-illumination system. It isunderstood that the illumination system may be constructed with obliqueillumination. Further, as an illumination light source 1, for example, aDUV (deep ultraviolet rays) laser (for example, KrF excimer laser=248nm, ArF excimer laser=198 nm, etc.) is used. As described, a DUV (deepultraviolet ray) laser beam has a shorter wavelength and hence a highresolution, so that an optical image based on scattered light ordiffracted light from a defect such as submicroscopic foreign particleshaving a dimension of 0.1 μm or less etc. can be obtained.

Therefore, an illumination system 902 is constructed with: anillumination light source 910, such as a DUV laser; a polarizationcontrol optical system for setting a polarization condition of the laserbeam 104; a pupil scan illumination optical system 904 consisting ofeither one of the above-described first to fifth embodiments forscanning a laser beam over the pupil 917 of the objective lens 122; anda half mirror (1)121. A basic construction of a detecting optical system900 comprises: the objective lens 122; an imaging lens 912; a magnifyingoptical system 913; a polarization detecting optical system 914 forsetting a polarization condition of detected light in front of an imagesensor 915(801) and the image sensor 915(801) having DUV quantumefficiency of around 10% or more. By the way, the polarization detectingoptical system 914 in the detecting optical system 900 is used to shieldspecula reflected light (zero-th order diffraction light) from theobject (sample object) 204 and can be constructed with a spatial filterinstead. In this case, instead of the polarization detecting opticalsystem 903 in the illumination optical system 902, it is necessary toprovide, for example, a ring-zone illumination optical system with theuse of light sources arranged in an orbicular zone around a central axisof the optical system (secondary light source).

Further, a half mirror (2)921 is disposed on a detection optical path,and an automatic focusing system 922 is provided for adjusting a surfaceof the sample object 204 on a focus of the objective lens 122. Further,a half mirror (3)931 is disposed so as to construct an optical systemcapable of observing a position of the pupil of the objective lens 122with a lens (1)932 and a pupil observation optical system 933. Further,a half mirror (4)941 is disposed so as to construct an optical systemcapable of observing and aligning a pattern on the sample object 204with a lens (2)942 and an alignment optical system 943.

Consequently, a DUV laser beam emitted from the illumination lightsource 910 is converted into linearly polarized light by, for example,the polarization control optical system 903 and is scannedtwo-dimensionally over the pupil 917 of the objective lens 122 forperforming irradiation by the pupil scan illumination optical system904. The reflected light from the sample object 204 is transmittedthrough the half mirror (1)905 after passing through the pupil 917 ofthe objective lens 122

and forms an enlarged optical image of the sample 204 on the imagesensor 915(801) through the imaging lens 912 and the magnifying opticalsystem 913. By the way, the image sensor 915(801) can be configured todetect an image formed only with scattered light or a component ofdiffracted light from the surface of the sample object 204 by shieldinga linearly polarized component of the specula reflected light (zero-thorder diffraction component) from the sample object 204, for example, bymeans of the polarization detecting optical system 914.

Next, referring to FIG. 30, a signal processing system is described.That is, the signal processing system is composed of: an AD convertercircuit 402 for analog-to-digital conversion of an image signalrepresented by grayscale values formed by accumulation of signals fromeach column of pixels obtained from the image sensor 915(801), which iscomposed of sensor elements having DUV quantum efficiency of around 10%or more, for example, a TDI image sensor, in synchronization withtranslation of the test object 204 in the Y-direction; a delay memory403 for delaying a digital image signal outputted from the AD convertercircuit by an amount corresponding to, for example, 1 chip (or pluralpitches) of a circuit pattern repeated in the Y-direction; a comparatorcircuit 404 for extracting a signal, for example, a difference imagesignal, by comparing a digital detected image signal 408 obtained fromthe above-mentioned AD converter circuit 402 and a digital referenceimage signal 409 which was delayed by, for example, the amount of 1pitch through the delay memory 403, and binarizing this extracteddifference image signal using a predetermined threshold value to form abinarized image signal indicating defect candidates, such as foreignparticles, a circuit pattern defect, etc.; a feature quantity extractingcircuit 405 for extracting a feature quantity of a defect candidate,such as an area, location coordinates, a maximum length (for example,projected lengths to the X-axis direction and the Y-axis direction),moment, etc. based on the binarized image signal obtained from thecomparator circuit 404 indicating defect candidates, such as foreignparticles etc.; and a defect judging circuit 406 for judging a defectcandidate as a defect when the feature quantity of the defect candidateextracted by the feature quantity extracting circuit 405 surpasses apredetermined criterion. By the way, regarding the feature quantity, ata time when a certain feature is identified as a defect candidate, agrayscale value based on a digital detected image signal obtained fromthe AD converter circuit 402 may be added to an original featurequantity to extract a three-dimensional feature quantity.

Especially, in order to detect a defect such as submicroscopic foreignparticles of a size of around 0.1 μm or less etc., it is necessary toremove noise components, due to minute irregularities of the surface andthe underlying pattern of the test object 204, out of the signal toprevent erroneous detection. To this end, a real submicroscopic defect,such as foreign particles etc. can be detected through the steps of:extracting any difference image signal surpassing a predeterminedthreshold as a defect candidate; and discriminating whether an extractedsignal is a true defect, such as foreign particles etc., or falseinformation arising from a minute irregularity of the surface or anunderlying pattern according to the extracted feature quantity of eachdefect candidate.

According to the present invention, since a UV laser or DUV laser beamhaving a short wavelength can be used after its coherence is reduced, adefect of a circuit pattern having a pattern width as small as 0.2 μm orless can be detected with sufficient accuracy.

By virtue of the present invention, since high-illuminance UV lightemitted from a laser source can be used to irradiate a sample after itscoherence has been reduced, a higher-resolution image can be obtainedcompared to a case where conventional visible light is used asillumination light, and hence a defect can be detected with highsensitivity.

What is claimed is:
 1. An inspection apparatus comprising: a lightsource which emits an ultraviolet laser beam; an illuminating unit whichsequentially illuminates a plurality of points along a specimen withlight emitted from the light source by shifting the ultraviolet laserbeam periodically; a detection unit having a storage-type detector whichdetects an image at each point by accumulating detected light within aspecified period of time from each of the plurality of points which havebeen periodically illuminated sequentially along the specimen to obtaindetected images and further performs the summation of each detectedimage on the storage-type detector to acquire a summation imagecontaining each of the plurality of detected points on the specimen; anda processor which detects a defect on the specimen by comparing thedetected acquired summation image obtained by the detection unit with areference image.
 2. The inspection apparatus according to claim 1,wherein the illuminating unit illuminates sequentially the specimen withlight emitted from the light source periodically shifting theultraviolet laser beam on a field of view of the specimen.
 3. Theinspection apparatus according to claim 1, wherein the illuminating unitilluminates sequentially the specimen in synchronization with operationof the storage-type detector.
 4. An inspection method comprising: anilluminating step which sequentially illuminates a plurality of pointsalong a specimen with an ultraviolet laser beam by shifting theultraviolet laser beam periodically along the specimen to each point; alight detecting step for detecting an image with a storage-type detectorat each point by accumulating detected light within a specified periodof time from each of the plurality of points which have beenperiodically illuminated sequentially along the specimen to obtaindetected images; a performing step for performing a summation of each ofthe detected images on the storage-type detector to acquire a summationimage containing each of the plurality of detected points on thespecimen; and a defect detecting step for detecting a defect on thespecimen by comparing the acquired summation image with a referenceimage.
 5. The inspection method according to claim 4, wherein theilluminating step periodically shifts the ultraviolet laser beam on afield of view of the specimen.
 6. The inspection apparatus according toclaim 4, wherein the illuminating step performs in synchronization withoperation of the storage-type detector.
 7. The inspection apparatusaccording to claim 4, wherein the illuminating step performs within astorage time of the storage-type detector.